As far as I know (and I'm no expert), you have to do your phase 1a at a regional training centre. 6 Weekends every other week. You can do extra prep work at your TAC for the weekends to make sure you are up to scratch.

If you are joining a unit such as the HAC or 4 Para then they do their selection and training independantley of RTC's. However if not you will have to attend selection at one of the ADSC's and then complete Ph1A/B at an RTC as steady state training (weekends) or on one/two week package when available.